Alumina, mullite, and other refractory ceramics have been used for some time as substrates for printed circuits having conduction patterns made from inks or pastes containing tungsten, molybdenum, gold, silver, or copper powders, terminal pads to attach semiconductor chips, connecting leads, capacitors, resistors, covers, and vias (i.e. holes filled with metal paste) to connect different layers of conductive patterns. Alumina is preferable due to its excellent electrical insulating properties, thermal conductivity, stability, and strength. However, this material has some disadvantages including signal propagation delay and noise in high performance applications, restriction of the type of the conductive metals due to alumina's high maturation temperature, and a high thermal expansion coefficient.
As an alternative to alumina, U.S. Pat. No. 2,920,971 to Stookey discloses the use of glass ceramics with dielectric properties and high mechanical strengths. Stookey's glass ceramics are characterized as "bulk-crystallized" or "bulk" glass ceramics as opposed to sintered glass ceramics. Bulk glass ceramics are generally inferior to sintered glass ceramics due to the latter's high flexural strength.
Although sintered glass ceramics are well known, they are generally unsuitable as substrates for printed circuits, because many glass ceramics can only be sintered at temperatures well in excess of 1000.degree. C. Such temperatures are above the melting point of the gold (having a melting point of 1064.degree. C.), silver (962.degree. C.), and copper (1083.degree. C.) conductors within the printed circuit.
U.S. Pat. No. 4,587,067 to Agrawal et al. discloses a method of manufacturing low thermal expansion-modified cordierite compositions. The process comprises: Blending MgO, Al.sub.2 O.sub.3, SiO.sub.2, and GeO.sub.2 ; Homogenizing the blended material by wet ball milling; Oven drying; Comminuting; Cold compacting; Sintering; and Consolidation to full density. Sintering is carried out at about 1350.degree. C.
One way of sintering below 1000.degree. C. is by treating the glass powder with an alkali solution and then sintering under vacuum, as taught by Helgesson, Science of Ceramics, British Ceramic Society, 1976, pp. 347-361. Another technique of sintering below 1000.degree. C. is to use glass compositions which are unsuitable as substrates for printed circuits due to their relatively high fluidity. This fluidity causes movement of the substrate's conduction patterns when sintering these materials.
U.S. Pat. No. 4,413,061 to Kumar, et al., ("Kumar") seeks to overcome the above-noted problems with respect to sintered glass ceramics by crystallizing the glass ceramics during sintering so that a rigid network of crystallites are formed. These crystallites reduce the fluidity of the substrate, thereby permitting greater dimensional and distortional control. Cordierite (2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2) glass ceramics according to Kumar are prepared by melting a mixture of Al.sub.2 O.sub.3, MgO, SiO.sub.2, and other materials, quenching the molten glass by pouring it into cold water to produce a cullet, grinding the cullet, mixing the ground cullet with a binder, casting the mixture into sheets, placing conductive patterns on the sheets, laminating, and sintering usually at temperatures of at least 925.degree. C. (see Table III). As to one example (i.e. Example 10), sintering temperatures as low as 870.degree. C. are indicated to be satisfactory. The resulting cordierite is primarily in the .mu. or .alpha. phases or mixtures thereof. Although Kumar produces cordierite, the grains of this cordierite are not homogenous. Kumar also requires a very high temperature glass melting step and a fairly high temperature sintering step. Moreover, it is difficult to achieve uniform fiber reinforcement when the fibers are mixed with Kumar's ceramic powder prior to sintering.
Ceramic materials have been formed from alkoxides. However, it has not been possible to manufacture certain ceramic compositions from a homogeneous liquid magnesium-aluminum-silicon alkoxide in a single reactor, because the magnesium alkoxides are generally insoluble in common organic solvents. Even if each of the alkoxides are separately formed and then mixed, a uniform distribution cannot be achieved due to magnesium's insolubility.
U.S. Pat. Nos. 4,242,271 and 4,288,410 to Weber, Hill, and Weeks prepare aluminum alkoxides by reacting impure aluminum with a stoichiometric excess of monohydric alcohol.
U.S. Pat. No. 3,761,500 to Thomas ("Thomas") discloses a magnesium, aluminum double alkoxide and the process of preparing it according to the following non-catalytic reaction: EQU Mg+2Al(OR).sub.3 +2ROH.revreaction.Mg[Al(OR).sub.4 ].sub.2 +H.sub.2 +MgAl.sub.2 (OR).sub.8
where the OR group is an alkoxy group of 4-7 carbon atoms. Thomas' process involves the reaction of two equivalents of aluminum alkoxide with one equivalent of magnesium. Consequently, the composition of the final product is limited to a ratio of magnesium to aluminum of 1 to 2. The double alkoxides can be used to form ultra high purity spinel by hydrolyzing the double alkoxide with water and then calcining or firing the hydrolysis product (col. 2, lines 33-41). The product of this process is useful as a refractory material.
U.S. Pat. No. 3,791,808 to Thomas relates to a process of preparing a thermally crystallizable oxide product by hydrolyzing a silicon alkoxide, reacting the hydrolyzed product with a metal alkoxide, an aqueous metal solution, or water to produce a gel, and heating to produce a thermally crystallized product. The thermally crystallized product is preferably a particulate mass with a particle size less than about 0.2 micron which can be treated to produce either a dense or porous body. Although any metal is said to be suitable in the metal oxide component, magnesium cannot be used in the alkoxide form, because such alkoxides are not suitably soluble. The products disclosed by U.S. Pat. No. 3,791,808 are useful in heat exchangers, dinnerware, and filters.
U.S. Pat. No. 4,052,428 to Lerner, et al., relates to the preparation of stable aluminum alkoxide solutions by reacting aluminum metal with a mixture of isobutyl alcohol and n-butyl alcohol. The resulting alkoxide is useful in forming catalysts and paint additives.
U.S. Pat. No. 4,266,978 to Prochazka discloses a non-aqueous gel of at least two metal oxides which are prepared by reacting a metal alkoxide with a metal halide and heating the reaction product. The gel is calcined at a temperature from 600.degree. C. to about 1300.degree. C. to produce a glassy or crystalline submicroscopically homogeneous mixture of the oxides.
U.S. Pat. No. 4,422,965 to Chickering, et al., discloses a process for containing a solution of nuclear waste in borosilicate glass. The glass precursor is prepared from a mixture of alkoxides which are hydrolyzed and then polymerized into an organic-free oxide network.
U.S. Pat. No. 4,430,257 to Pope, et al., relates to a method of containing nuclear waste in a glass forming composition prepared from tetraethylorthosilicate (TEOS), an aluminum alkoxide, or a magnesium-aluminum alkoxide in a 1 to 2 ratio, a boron alkoxide or a calcium alkoxide, an alcohol, and a sodium compound. Once the composition components are intimately mixed, excess alcohol and water are boiled off as an azeotrope to leave a homogeneous colloidal glass forming composition.